In recent years, fiber optics have formed the basis for many different types of sensors, such as pressure sensors, strain sensors, and others. Optical fiber sensors can use phase, polarity or intensity modulation. Intensity modulation yields fiber optic sensors that are simpler and less expensive. The theory of intensity modulated optical fiber sensors, and examples of such sensors, are disclosed in the U.S. Government-owned inventions by Nicholas Lagakos et al., described in U.S. Pat. No. 7,020,354: Intensity Modulated Fiber Optic Pressure Sensor, Navy Case No. 83,816; U.S. Pat. No. 7,149,374: Fiber Optic Pressure Sensor, Navy Case No. 84,557; U.S. Pat. No. 7,379,630: Multiplexed Fiber Optic Sensor System, Navy Case No. 97,488; U.S. Pat. No. 7,460,740: Intensity Modulated Fiber Optic Static Pressure Sensor System, Navy Case No. 97,279; U.S. Pat. No. 7,646,946: Intensity Modulated Fiber Optic Strain Sensor, Navy Case No. 97,005; U.S. Pat. No. 7,697,798: Fiber Optic Pressure Sensors and Catheters, Navy Case No. 97,569; U.S. Pat. No. 8,195,013: Miniature Fiber Optic Temperature Sensors, Navy Case No. 98,030 (collectively, the “U.S. Government Patents”). The disclosures of the U.S. Government Patents are incorporated herein by reference.
Electrical power monitoring and control systems require specialized sensor devices. The requirements associated with such sensors are determined by the particular monitoring and control approach employed, as well as the kind of performance expected of the monitoring or controlling system. A common type of electromagnetic sensor are instrument transformers, which measure current and voltage in a circuit when the electrical potential or current is too high to be applied directly to the measuring instruments. These transformers operate to reduce the current or voltage that is proportional to the current or voltage in the circuit, which can then be connected to measuring and control instruments. While electro-mechanical instrument transformers have been in use in electric power systems for many years, the accuracy of these devices is limited by a number of factors which impose significant measurement errors in actual use. In addition, these devices are often physically complex, are sensitive to electromagnetic interference (EMI), can be dangerous to humans in operation, and aren't physically robust.
One solution is to employ measurement devices that use fiber optics for measurement. The use of fiber optics for sensors in general, and for electromagnetic measurements in particular, is an alternative that addresses the accuracy, EMI sensitivity, safety, size, and robustness concerns inherent in existing electro-mechanical electric system measurement. Since fiber optics use light rather than electricity, a fiber optic sensor is generally insensitive to EMI and is therefore more efficient in an environment that has a large amount of electromagnetic energy. As a result, fiber optic sensors can be located adjacent to or attached to circuits that generate large electro-magnetic fields without negative effects to either the measurement or the measuring equipment.
One known approach has been to use fiber optic sensor devices to measure changes in light phase and polarity that indicate various electric and magnetic phenomena that are produced by the Farraday, Kerr, and Pockels effects. See, e.g., U.S. Pat. Nos. 7,714,735; 7,786,719; 7,327,512; 6,621,258; 5,631,559; and 5,400,142. These techniques rely upon mechanisms by which electromagnetic properties in an area of interest can be measured by observing the interaction between light and an electromagnetic field in a given material, such as a length of fiber optic cable. The rotation of the plane of polarization of light passed through an optical fiber subjected to an electromagnetic field is indicative of the intensity of that field. However, performance of these devices is limited by the accuracy with which the change in polarization of the light can be measured, which requires specialized light polarization and filtration devices that must remain properly calibrated, which raises the complexity and lifetime costs associated with these devices. These devices also are sensitive to temperature effects.
Intensity modulated fiber optic sensors are an alternative means of achieving accurate measurement of electromagnetic phenomena that have the benefit of being simpler and less expensive than existing methods. Therefore it is an object of this invention to offer an intensity modulated fiber optic electromagnetic phenomena sensor that is: highly accurate; may be used in areas with high potential for EMI; physically robust; and physically simple.